Agricultural sprayers typically have long spray booms with groups of spray nozzles that are selectively controlled by respective control valves, each of which is individually actuated to permit transmission or termination of liquid to the spray nozzles of the group. Termination of liquid to one or more groups of spray nozzles frequently is necessary, for example, near the ends of fields so as to prevent spraying on non-crop vegetation or the like. The control valves for such spraying systems commonly are mounted in ganged or manifolded side-by-side relationship with respect to each other and remote from the spray nozzles they control. Each control valve is provided with a valve element that is movable between open and closed positions to selectively control the flow of liquid to the spray nozzles of the respective group.
A problem associated with such agricultural sprayers is the tendency for fluid to drip or dribble from the nozzles after the supply of pressurized fluid has been shut off by the control valve. In an effort to prevent such drippage, each spray nozzle may be provided with a respective spring-actuated check valve, as shown in commonly assigned U.S. Pat. No. 4,660,598. Following movement of the control valve to a closed position and the ultimate reduction of line pressure to a level below the check valve spring pressure, the check valve will snap closed to prevent further liquid from entering the spray nozzle. If the nozzle discharge orifice is relatively large, following closure of the control valve, line pressure downstream of the control valve will quickly drop below the check valve spring pressure by reason of relatively quick run out through the nozzle. If the discharge orifice is small, and particularly at high line pressures, it can take a considerable time, following movement of the control valve to a closed position, before the line pressure reduces to a level that permits the check valve to close. In either case undesirable dripping and dribbling can occur. Since spray booms can carry 60 or more nozzles, this can result in costly waste of chemicals, the over concentrated application of chemicals, and the direction of chemical to unwanted areas. While proposals have been made for more quickly shutting off such check valves following movement of the control valve to a closed position, such as by use of air pressure or other auxiliary check valve moving means, those proposals have been relatively complex and expensive.